1. Field of the Art
The present invention relates to a process for producing an inorganic oxide organosol comprising inorganic oxide particles, to silicon atoms on the surface of which primary alkoxy groups with 3 to 12 carbon atoms being bonded, being stably dispersed in a organic solvent.
2. Description of the Related Art
There are proposed processes for producing inorganic oxide organosols (inorganic oxide sols dispersed in organic solvents) as follows:                U.S. Pat. No. 2,801,185 discloses a colloidal silica which can be re-dispersed in an organic solvent, has a mean particle diameter of 5 to 150 nm, hydrophobic by alcohol groups or organosilyl groups chemically bonded to the surface of the silica, and has hydroxy groups on the surface of 10 m2 or less per 1 g measured by Methyl Red adsorption method. In the patent, dimethyl dichlorosilane is added to a silica sol containing triethyl phosphate as a dispersion medium, and after the reaction hydrochloric acid, solvent, excess dichlorosilane and the like are distillated off to give a solid that can be dispersed in benzene or chloroform;        JP-A-57-196717 (1982) discloses a silica powder which the surface thereof is esterified and can be dispersed in an organic solvent, and which is obtained by heating a silica sol dispersed in an alcohol with 2 to 18 carbon atoms at a temperature of 170 to 300° C. and then distilling off the alcohol;        JP-A-58-145614 (1983) discloses a silica powder which can be re-dispersed in an organic solvent, and which is obtained by adding a silylating agent to an organosilica sol having a water content of 10% or less, reacting them each other and then distilling off the solvent and in which silyl groups with 1 to 36 carbon atoms are bonded to the surface of colloidal particles in a rate of 1 to 100/10 nm2;        JP-A-3-187913 (1991) discloses a silica powder which the surface thereof is silylated and can be re-dispersed in a solvent, and which is obtained by adding a trimethyl silylating agent to a sol dispersed in methanol obtained by hydrolyzing an alkyl silicate in methanol in an amount of 5 mol % or more of the trimethyl silylating agent based on the silica, reacting them each other, and then distilling off excess trimethyl silylating agent and the solvent for dispersion;        WO 96/34063 discloses a process for producing a sol comprising azeotropically dehydrating water contained in an aqueous inorganic oxide sol by use of an azeotropic solvent with water, and then surface-treating with a silane coupling agent. Further, the publication discloses that the solvent can be replaced with a desired one, if required;        JP-A-11-43319 (1999) discloses a process for producing a silica sol dispersed in a hydrophobic solvent comprising silylating in a mixed solvent of a hydrophilic solvent with a hydrophobic solvent, and then replacing solvent; and        JP-A-2001-213617 (2001) discloses a process for producing a hydrophobic colloidal silica comprising adding a hydrolyzable silicon compound having one or more alkyl groups in the molecule to a colloidal silica containing a hydrophilic organic solvent as a main solvent to obtain a treated colloidal silica, and subjecting the treated colloidal silica to solvent replacement with ultra filtration membrane to produce a hydrophobic colloidal silica containing a hydrophobic organic solvent as a main solvent.        
It is known a process for making an inorganic oxide sol organophilic by directly reacting an alcohol with hydroxyl group (OH group) on the surface of inorganic oxide particles. However, the process generally requires a reaction at a high temperature. In order to react an alcohol having a low boiling point, it is required to heat in a pressure vessel such as an autoclave. When an alcohol having a high boiling point is reacted, it can be reacted at atmospheric pressure (1 atm). However, it is difficult to produce a sol stably dispersed in an alcohol having a high boiling point before the surface treatment and to delete unreacted alcohol after the reaction.
In addition, inorganic oxides except silica are generally subject to hydrolysis of alkoxy groups, and therefore it is difficult to maintain long-term stability.
It is also carried out a process for introducing silyl groups and alkoxy groups by reacting trialkoxy silane and the like to the surface of inorganic oxide particles. However, methoxy silane or ethoxy silane that is generally used does not afford organophilic properties or dispersion stability adequately, the alkoxy groups are susceptible to hydrolysis, and sols having a long-term stability are not obtained.
When a silicon alkoxide is reacted with an inorganic oxide in an alcohol having a boiling point of 100° C. or more in conventional processes, it is difficult to delete the alcohol, and therefore a large amount of alcohol tends to remain even after solvent replacement with a hydrophobic solvent, and the alcohol is apt to cause adverse effects in the use scene of the sol. Further, the untreated hydrophilic inorganic colloid is difficult to disperse in an alcohol having a high boiling point.
Although a sol to which trialkyl silyl groups such as trimethyl silyl group are bonded has a high shelf stability, only a few kinds of trialkyl silylation agents can be available, and therefore it is difficult to alter surface treating groups. In addition, reactivity with inorganic oxides other than silica is low, and it is difficult to adapt this process. Further, sols obtained by treating with a trialkyl silyl group have problems that they have poor bonding force or poor reactivity with resin binder.